In Eristische Dialektik, published after his death and widely read as Die Kunst, Recht zu behalten, the art of being right, Arthur Schopenhauer lists tricks for winning an argument. The last one is to go personal. Beim Persönlichwerden aber verlässt man den Gegenstand ganz, und richtet seinen Angriff auf die Person des Gegners: man wird also kränkend, hämisch, beleidigend, grob. In going personal, one leaves the subject entirely and directs the attack at the opponent’s person. One becomes hurtful, spiteful, insulting, rude. German Wikipedia summarizes: the argument against the person is a way out when the opponent seems to be winning.